Keratinocytes stratum granulosum

The stratum granulosum is the most superficial cell layer of the viable epidermis and contains highly differentiated keratinocytes. The lamellar bodies, which have been formed in the stratum spinosum, migrate to the apical periphery of the uppermost granular cells and eventually fuse with the membrane of the keratinocyte. Via exocytosis their content is extruded into the intercellular spaces at the stratum granulosum-stratum corneum interface. The lipids derived from the lamellar bodies are essential for the formation of the stratum corneum barrier. 

Diffusion of aqueous material through the epidermis is blocked by these lipids synthesized by keratinocytes in the stratum granulosum, the boundary corresponding to the level at which HA-staining ends. This constitutes part of the barrier function of skin. The HA-rich area inferior to this layer may obtain water from the moisture-rich dermis. And the water contained therein cannot penetrate... 

The epidermis consists of five principal layers and is an area of both intense biochemical activity and differentiation. These layers are the stratum comeum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The stratum corneum (horny layer) is the uppermost layer of the epidermis and the skin. The stratum corneum is composed of dead keratinocytes, which are called corneocytes, and has an abundance of keratin and lipid structures [8], The stratum comeum is considered the rate-limiting barrier for the diffusion of chemical compounds across the skin. The stratum lucidum (clear layer) is composed of two to three layers of dead flattened keratinocytes which appear translucent under a microscope and are present only in thick glabrous skin. 

Human keratinocytes express cholinergic receptors in the cells of the stratum basale, the stratum spinosum and the stratum granulosum. They use ACh, among other things, to stick together. They also synthesize, store, degrade and release ACh. 

Keratinocytes of the epidermis are arranged into four or five layers. The deepest layer is the stratum basale, which is a single layer of dividing cells. The stratum spinosum is next and consists of 8-10 layers of polyhedral keratinocytes. Above this layer is the stratum granulosum, composed of three to five layers of keratinocytes that have lost their... 

The main cells of the viable epidermis are keratinocytes. They get their growth factors and nutrients by passive diffusion via the interstitial fluid, which is estimated to represent about 15% of the total volume of the epidermis and drains into the lymphatic system. The viable epidermis also contains melanocytes, Langerhans cells, migrant macrophages, and lymphocytes. The top two layers of the viable epidermis, the stratum lucidum and the stratum granulosum, are physiologically very important. Removal of these three epidermal layers results in water loss and an enhancement of skin permeability [8],... 

The epidermis is an epithelium consisting of inner viable epidermis, a living hydrophilic layer, and outer nonviable epidermis, a hydrophobic layer made from dead cells. It is differentiated into stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale In the direction of dermis (Fig. 16.2). The viable epidermal layer has a thickness of about 0.02 to 0.2 mm. It is composed of many layers of keratinocytes, a widespread distribution of melanocytes, Langerhans cells, dendritic T cells, epidermotropic lymphocytes and Merkel cells, and a number of catabolic enzymes such as esterases, proteases, phosphatases, nucleotidases and lipases [Walters and Roberts, 2002 Barry, 2001). The outer and nonviable epidermis, namely, stratum corneum, is about 10 to 20 pm thick [Gregor and Ulrich, 2010). The stratum corneum Is deemed to be the major obstacle of drug permeation. 

The corneocytes are embedded in a complex lamellar lipid matrix forming 20% of the volume of the SC (Forslind 1994) and 8-10.3% of its total dry weight (Schurer et al. 1991 Martini 1995). The main epidermal lipids are the ceramides ( 49%), fatty acids ( 26%) and cholesterol ( 20%) (Forslind 1994). Within the deeper keratinocytes, these lipids are collected in the epidermal lamellar bodies or Odland bodies, already disposed in lamellae, and are then discharged to the extracellular space at the upper layers of the stratum granulosum. They are synthesized either de novo or from rearrangement of polar lipid precursors, as... 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

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